Protective devices are used to protect the power lines and power system equipment from damage caused by excessive voaltages and currents. One type of protective device is a fuse that functions to open a circuit when electric current in the circuit exceeds a predetermined level. A fuse cutout is a special kind of fuse that is used to protect equipment from burn out caused by heavy instantaneous fault currents or continuous overcurrents. The principle of the fuse cutout is essentially the same as that of the simple fuses found in the home. A small piece of soft conducting metal which melts at a very low temperature is placed in series with the line current so that an overload will heat the link causing it to break and form an open circuit.
In a power system, however, the current magnitude is so great that a more elaborate apparatus is needed to prevent violent and continuous arcing across the open contacts. Typically, an apparatus is used whereby the arc is extinguished and the distance between the contacts is greatly increased to prevent restrike. The fuse link itself, supported at both ends by a flexible pigtail conductor, is placed inside a tough fiber-lined tube. The lower end of the pigtail is sets a flipper latch spring and is wound around a bolt and secured by a nut. During a fault, the fuse link melts and a momentary high temperature arc results. A portion of the fiber lining then vaporizes forming a deionizing gas which dilutes and cools the conducting path of the arc with small non-conducting particles. The arc is then extinguished at the first current zero.
When the arc is disposed of, the flipper latch first prevents restriking by pulling the lower half of the broken pigtail out the bottom of the fuse tube. It is helped in this process by the exhaust blast of gases which tend to carry the link along. Second, the latch releases a toggle mechanism which drops the fuseholder straight down, freeing it from the upper contact. This allows the fuseholder to swing to a full 180 degree open position, providing easy identification of a fault interruption and also providing an added safety measaure against restriking. However, when the fuseholder swings down, it is difficult to detect from a viewing distance whether the fuse is electrically alive or dead. Accordingly, it will be appreciated that it would be highly desirable to have a fuse assembly that gives a highly visible indication of its electrical condition.
Because the fuse is positioned in the circuit to protect certain portions of the line and certain equipment, some fuses are placed in locations remote from the roadway. It is common for a pole mounted fuse to be mounted on the pole wherever the pole may be, whether it is a wooded area or an open field. It is common for a power line to run parallel or almost so with a roadway because houses that use electricity are located near the roadway. It is not uncommon when a workman is looking for the fuse that has operated to search for the fuse when it is dark because overcurrents occur at night, as well as, in the daytime. Overcurrents often occur during a thunderstorm. Weather conditions are seldom perfect when the workman is looking for the fuse that has operated. Thus, a workman may drive along the road and use a flashlight shined upon the assembly to locate the operated fuse.
An operated fuse can be identified by the position of the fuse tube. Unfortunately, the position of the fuse tube is difficult to see at night with only the aid of a flashlight, or in daylight after exposure to dirt and other contaminants. Accordingly, it will be appreciated that it would be highly desirable to have a fuse tube that is easy to see in all weather conditions and especially at night.